search: return a result per branch in indexed results

Created by: keegancsmith

If a file is the same in two branches and we have results on it, our indexed search currently will only return one result. This would be misleading in the UI since the user would not know the other branches have results.

This change will copy how we do it for non-indexed search and just duplicate results (but with different input revs). In future we should dedup results and display this information more compactly.

Note: This will be cherry-picked onto the 3.18 release branch.